(1) Field of the Invention
The present invention relates to a cable broadcasting system by which an image signal is broadcast by an on-demand-video service method through a cable network such as a public network of Broadband Integrated Service Digital Network (B-ISDN) or a cable television (CATV) and a transmission center employed therein.
(2) Description of the Related Art
In a cable broadcasting system based on a cable network, such as represented by the CATV, a program is broadcast according to a timetable for each channel like in a wireless broadcasting service. Subscribers of the cable broadcasting system choose a program according to a time slot and a channel thereof specified in the timetable, although only a few dozen channels are available due to bearer capability.
Recently, a new service method taking the advantage of the two-way method of the CATV, i.e. a pay-TV or a pay-per-view service, has been in practical use, in which a program is provided only to the subscribers who have requested the program. By such a method, although various programs and accompanying services are available for a charges, the subscribers still have to choose their program according to a time slot d a channel as specified in the timetable.
It is therefore the subscribers who arrange their schedule to the program they would like to watch, and when they missed the program or would like to watch it again, they have to wait for a repeat. Thus, a limited number of the available channels provide the subscribers limited options to a program selection and a less flexible schedule. In brief, the subscribers can neither choose a program among various kinds or programs nor watch it at their convenience with the conventional cable broadcasting system. Therefore, and on-demand-video service method is proposed in order to add the options, by which a program selected from a large program library in a broadcasting station is transmitted to the subscribers at their request.
Such a video distribution system capable of answering the subscribers' request is proposed in "FM Multiplex High-Vision Optical CATV System by a Demand-Access Method", by Mikio Maeda Nippon Hoso Kyokai Giken, Shingakugiho Vol. 89 No. 304 (OQE8 9 91-95) pp. 7-12, and "Alan F. Graves (Bellcore) `Switched digital video-The opportunities and challenges of Heathrow` Pro. National Communication Forum Vol. 43 No. 2 pp. 1125-1128."
According to the system, upon receiving the subscribers' requests for the programs, the broadcasting station selects each program and transmits it only to those who have requested it.
However, the system requires transmission line in a star network in order to connect the broadcasting station directly to each subscriber and a switch device for each subscriber for the program selection. Therefore, introducing the on-demand-video service method to the CATVs with existing transmission lines in a tree network requires a complete line exchange. Furthermore, more massive and complicated hardware will be necessary at the broadcasting station, thereby increasing service costs. In addition, since the system proposed in the aforementioned publication is supposed to be available to the timetable, the options in the program selection remain limited.